Clip stems from figs.
Using metal blade, process figs, a portion at a time, with on-off bursts until coarsely chopped; you should have 6 cups.
Place in a 6-quart or larger pan.
Peel, core, and quarter apples.
Process, a portion at a time, with on-off bursts until coarsely chopped; you should have 6 cups.
Add to figs.
Process garlic until chopped.
Cut onion in chunks, add to garlic, and process with on-off bursts until onion is coarsely chopped.
Add to figs.
Cut lemon into quarters and thinly slice; discard seeds.
Add to figs along with vinegar, brown sugar, red pepper, allspice, cinnamon, salt, and cloves.
Bring to a boil; cook, uncovered, over medium-low heat, stirring frequently until mixture thickens (35 to 40 minutes).
Meanwhile, process walnuts (if desired) with on-off bursts until chopped.
Stir into chutney.